A bottom package for a PoP (package-on-package) may be formed with a reinforcement layer supporting a thin or coreless substrate. The reinforcement layer may provide stiffness and rigidity to the substrate to increase the stiffness and rigidity of the bottom package and provide better handling of the substrate. The reinforcement layer may be formed using core material, a laminate layer, and a metal layer. The substrate may be formed on the reinforcement layer. The reinforcement layer may include an opening sized to accommodate a die. The die may be coupled to an exposed surface of the substrate in the opening. Metal filled vias through the reinforcement layer may be used to couple the substrate to a top package.